“Boom, Pregnant!” is a catchphrase or image caption most often found in response to a photo of a handsome man. The expression may have originated from “Boom Headshot!”, a phrase popularized by an episode of web mockumentary Pure Pwnage.
Origin
Prior to its use as commentary, “Boom, Pregnant” was used as early as March 20071 to describe a woman’s surprise at being pregnant. It was used to describe an attractive male as early as July 2010 in response to a photo of the lead singer of rock band Queens of the Stone Age, Josh Homme, on LiveJournal gossip blog Oh No They Didn’t.2

Notable Examples
Between September and December 2010, the phrase was used several more times on Oh No They Didn’t, describing actor Tom Hardy three times347, as well as actor Chris Pine.5 In January 2011, a Facebook fan page17 for the phrase was established, mostly for fans of the members of the British alternative band Muse. That April, the caption was first used on Reddit in /r/LadyBoners9 on a picture of actor Jensen Ackles. On August 19th, 2011, a single topic Tumblr titled “Boom Pregnant”6 was created, featuring photos of male actors and musicians. A second single topic Tumblr of the same name8 was launched in June 2012.
On Tumblr, “Boom, Pregnant” is commonly employed as a tag10 on posts of attractive people. However, the tag has several different iterations based on placement of punctuation including periods11 and exclamation points.1213 There is also a tag for “boom you’re pregnant.”14 As of October 1st, 2012, there are nearly 200 results for “boom pregnant” on Flickr15, however some of these include images of pregnant women. There are also more than 1,400 results for the phrase on YouTube.16